ALLUVIUM, the material deposited by rivers (see the gen eral article GEOLOGY). It is usually most extensively developed in the lower part of the course of a river, forming flood-plains and deltas, but may be deposited at any point where the velocity of a river is checked; for example, where it runs into a lake. Alluvium in the true sense consists mostly of silt, sand, and gravel, without much true clay; it often contains a good deal of organic matter and therefore yields very fertile soils. Some of the great alluvial deposits of the world are: the flood-plain of the Mississippi, the deltas of the Nile, of the Ganges and Brahmaputra, and of the Hwang Ho in China. Owing to elevation of the land and conse quent deepening of the river channel, patches of alluvium are often left forming terraces on the sides of valleys. On such ter races much of London is built. In some regions alluvial deposits contain gold, platinum or gemstones, and the greater part of the world's supply of tin ore comes from similar sources.
(R. H. RA.)